The simplest concept in compression is the idea that you average four nearby pixels together - if they're really similar? You can replace them with one piece of data (rather than the original four.)

Noisy footage happens at the source for two reasons: poorly lit and a very compressed starting point (I'm looking at you DSLR h.264 files!). The worse the starting point, the more important it is to shoot right - else you have to deal with noise, meaning it'll be harder to compress your footage.

Make sure to view this in full. the noise on the left hand side makes this file far harder to compress.